There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Electrochemistry represents one of the most intimate ways of interacting with molecules. This review discusses advances in synthetic organic electrochemistry since 2000. Enabling methods and synthetic applications are analyzed alongside innate advantages as well as future challenges of electroorganic chemistry.

Synthetic Organic Electrochemical Methods Since 2000: On the Verge of a Renaissance

C–H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C–H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C–H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C–H activation until summer 2018.

3d Transition Metals for C-H Activation.

Organic reactions that involve the direct functionalization of non-activated C–H bonds represent an attractive class of transformations which maximize atom- and step-economy, and simplify chemical synthesis. Due to the high stability of C–H bonds, these processes, however, have most often required harsh reaction conditions, which has drastically limited their use as tools for the synthesis of complex organic molecules. Following the increased understanding of mechanistic aspects of C–H activation gained over recent years, great strides have been taken to design and develop new protocols that proceed efficiently under mild conditions and duly benefit from improved functional group tolerance and selectivity. In this review, we present the current state of the art in this field and detail C–H activation transformations reported since 2011 that proceed either at or below ambient temperature, in the absence of strongly acidic or basic additives or without strong oxidants. Furthermore, by identifying and discussing the major strategies that have led to these improvements, we hope that this review will serve as a useful conceptual overview and inspire the next generation of mild C–H transformations.

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Mild metal-catalyzed C–H activation: examples and concepts

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure The schemes feature typical substrates used, the products obtained as well as the required reaction conditions Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them Accordingly, this review should be of particular interest also for scientists from industrial R&D sector Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date

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A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry.

Oxidative Coupling between Two Hydrocarbons: An Update of Recent C–H Functionalizations

Electrochemical Oxidative Cross-coupling with Hydrogen Evolution: A Green and Sustainable Way for Bond Formation

Direct olefinic C–H functionalization represents the ideal way of introducing an alkenyl group into organic molecules. A well-known process is the Heck reaction, which involves alkene insertion and β-hydride elimination in the presence of a transition metal. However, the traditional Heck reaction mainly deals with the alkenylation of aryl or vinyl electrophiles. Recent developments have revealed that alkenylation can also be achieved through radical addition to alkenes and following single-electron-transfer (SET) oxidation/elimination. The radical alkenylation pathway allows alkenylation with a variety of carbon-centered radicals and even heteroatom-centered radicals. This tutorial review gives an overview of recent advances in this emerging field.

Olefinic C–H functionalization through radical alkenylation

An environmentally friendly electrocatalytic protocol has been developed for dehydrogenative C−H/S−H cross-coupling. This method enabled C−S bond formation under catalyst- and oxidant-free conditions. Under undivided electrolysis conditions, various aryl/heteroaryl thiols and electron-rich arenes afforded the C−S bond-formation products in 24–99 % yield. A preliminary mechanistic study indicated that the generation of aryl radical cation intermediates is key to the success of this transformation.

Electrocatalytic Oxidant‐Free Dehydrogenative C−H/S−H Cross‐Coupling

An environmentally friendly electrochemical protocol about cobalt-catalyzed C–H amination of arenes has been developed, which offers a simple way to access synthetically useful arylamines. In divided cells, a wide variety of arenes and alkylamines are examined to afford C–N formation products without using external oxidants, which avoids the formation of undesired byproducts and exhibits high atom economy. Importantly, the reaction can also be extended to gram level with moderate efficiency. KIE experiments indicate that C–H bond cleavage might not be involved during the rate-limiting step.

Shan Tang

Papers

Oxidative Coupling between Two Hydrocarbons: An Update of Recent C–H Functionalizations

Electrochemical Oxidative Cross-coupling with Hydrogen Evolution: A Green and Sustainable Way for Bond Formation

Olefinic C–H functionalization through radical alkenylation

Electrocatalytic Oxidant‐Free Dehydrogenative C−H/S−H Cross‐Coupling

Cobalt(II)-Catalyzed Electrooxidative C-H Amination of Arenes with Alkylamines.